The film model is the simplest approach to the prediction of mass transfer coefficients. It is not as elegant as boundary layer theory, penetration theory or a variety of other theories used. With film theory you can transform a complex convective transfer problem into a simper soluble problem. The goal is to retain enough of the essential physics to ensure that the simple model will predict some of the observations of the complex system.
The major assumption is that all of the resistance to mass transfer occurs in an immaginary film of unknown thickness.
By making these assumptions the problem becomes easily solved because the parameter values can all be measured or looked up in a table.